Wood adhesives made from the combination of a polyamidoamine-epichlorohydrin resin (PAE resin) and soy protein are being used as alternatives to formaldehyde-containing adhesives such as urea-formaldehyde (UF) resins, phenol-formaldehyde (PF) resins and melamine-formaldehyde (MF) resins (U.S. patent application Ser. No. 10/438,147). Performance of the PAE-soy adhesives compares quite favorably to the formaldehyde-containing materials. The PAE/soy adhesive system has been successfully applied to the manufacture of hardwood plywood (Brown, Valerie J., “Better Bonding with Beans” in Environmental Health Perspectives, 113(8):A538-A541, 2005 and “Columbia Forest Products Launches a Revolution in Plywood Adhesives”, Environmental Building News, 14(6):9, 2005). However, use of a PAE or other formaldehyde-free crosslinker/soy adhesive system in other wood composite applications such as particleboard, medium density fiberboard (MDF), and oriented strandboard (OSB) requires a lower viscosity adhesive system.
For particleboard, MDF, and OSB, adhesive is generally sprayed onto the wood furnish, which requires a low viscosity adhesive system. Diluting the adhesive with more water to lower viscosity is a limited option, since adding too much water can cause steam blows in the press or require long press times to remove the excess water. For the soy/PAE system, one way to lower viscosity is to lower the viscosity/molecular weight of the PAE resin (U.S. patent application Ser. No. 11/895,122, US20080050602A1). Another way to get lower viscosity while maintaining solids is to use urea as a non-volatile denaturant/diluent to essentially lower the soy level (U.S. patent application Ser. No. 11/779,558, US20080021187A1). Thus, conventional systems utilize urea to provide good performance characteristics; however, a non-urea alternative would be desirable in the market place.